Appliance manufacturers have increased the number of programs they wish to provide for their machines such as clothes washers and dishwashers. This places severe demands on the timer designer due to the fact only 360.degree. rotation of the timing (program) cam is available without repeating and there is a practical minimum speed and minimum angular travel for effective switching. Therefore, these minimums determine the basic requirements for the timing cam. Thus, if 6.degree. are required for proper switching, only 60 steps are available and some of these represent start or stop positions so the actual number is appreciably reduced. The switching should be caused by relatively fast movement of the cam which indicates an impulse drive or an eccentric drive.
In most timer programs there are a number of steps used simply to mark time . . . , i.e. provide time for some function. This suggests the desirability of reducing the speed of the program cam while no switching is required so more steps are available for switching. The prior art has addressed the problem of providing for two speeds of the timing cam--a fast speed for switching and a slow speed for marking time.
U.S. Pat. No. 3,882,734 shows a two-speed drive in which the gear train must be externally shifted to change speed. This requires many gears, clutches, ratchet devices, levers, and springs which add to parts cost and manufacturing cost. Only one speed is available at one time. The design is expensive and limited in use.
U.S. Pat. No. 3,857,293 provides two drive pawls each acting on its own ratchet on the periphery of the timing cam. One pawl moves only a short distance and engages a ratchet in which the teeth are small and closely spaced. The other pawl moves a longer distance and engages large teeth on the other ratchet. A stop (anti-reverse) pawl engages the finer teeth. The number of small teeth must be a multiple of the large teeth and the pawls must be operated synchronously to keep the switching synchronized, the idea being that if there is a large tooth missing, the pawl operating on the small teeth will have to advance the cam the number of small steps (teeth) necessary to equal a large tooth and bring a large tooth into range of the large pawl. This arrangement requires many parts and gears to get the necessary different operating speed and travel of the two pawls. This design is limited by its need for a large tooth to be some multiple of the small teeth and there is a practical limit imposed on the ratio of high speed to low speed given the fact that use of large angular travel is self-defeating (by using up too much of the available angular travel) and the small teeth get too small as the ratio increases.